1. Field of the Invention
The present invention is directed to low surface energy polyisocyanates which contain allophanate groups, siloxane groups and optionally isocyanurate groups, to a process for their preparation by allophanatizing the isocyanate groups of organic diisocyanates in the presence of isocyanate-reactive compounds containing siloxane groups, and to their use in one- and two-component coating compositions.
2. Description of the Prior Art
Polyurethane coating compositions containing a polyisocyanate component, in either blocked or unblocked form and an isocyanate-reactive component, generally a high molecular weight polyol, are well known.
Although coatings prepared from these compositions possess many valuable properties, one property, in particular, which needs to be improved is the surface quality. It can be difficult to formulate coating compositions to obtain a coating having a smooth surface as opposed to one containing surface defects such as craters, etc.
It is believed that these difficulties are related to the high surface tension of the two-component coating compositions. Another problem caused by the high surface tension is the difficulty in cleaning the coatings. Regardless of their potential application area, there is a high likelihood that the coatings will be subjected to stains, graffiti, etc.
Accordingly, it is an object of the present invention to provide coating compositions which have reduced surface tension and, thus, are suitable for the production of coatings having lower surface energies and improved surfaces. It is an additional object of the present invention to provide coating compositions which have improved cleanability. It is a final object of the present invention to provide coating compositions which satisfy these requirements without substantially affecting the other valuable properties of the known polyurethane coatings, in particular, their clarity.
Surprisingly, these objectives may be achieved by formulating coating compositions with the polyisocyanates according to the present invention containing allophanate groups, siloxane groups and optionally isocyanurate groups which are described hereinafter.
U.S. Pat. No. 4,590,224 is directed to the production of fully reacted polymers, primarily in form of molded articles or foams, which are prepared by reacting a polyisocyanate with a polysiloxane polyahl in the presence of a trimerization catalyst. In addition to the oligomerization of the polyisocyanate to form isocyanurate groups, a portion of the isocyanate groups with react with the polyahl to form urethane and urea groups depending upon the type of polyahl. This patent does not suggest the incorporation of allophanate groups into the polyisocyanate component and certainly does not recognize the importance of incorporating allophanate groups to maintain the clarity of polyurethane coatings.
The incorporation of fluorine into either polyisocyanates containing allophanate groups and isocyanurate groups or polyisocyanates containing allophanate groups to reduce the surface tension of the polyisocyanates and the surface energy of the resulting polyurethane coatings is disclosed in application, U.S. Ser. No. 08/306,553, filed Sep. 14, 1994, and in U.S. Pat. No. 5,541,281. Similarly, the incorporation of siloxane groups via allophanate groups using monoalcohols to reduce the surface tension of the polyisocyanates and the surface energy of the resulting polyurethane coatings is disclosed in application, U.S. Ser. No. 08/536,556 filed Sep. 29, 1995, U.S. Pat. No. 5,574,122.